fix(styles): resolve design system audit violations and touch scrolling

- Replace hardcoded px values with design tokens (--space-0h, --space-px,
  --space-1, --target-sm/md/lg) across layout, calendar, dashboard CSS
- Fix rgba(255,255,255,0.35) spinner border to use var(--color-glass-hover)
- Fix modal close icon from off-grid 18px to 16px
- Fix touch scrolling on mobile: add min-height: 0 and
  -webkit-overflow-scrolling: touch to .app-content

public/components/modal.js

@@ -195,7 +195,7 @@ export function openModal({ title, content, onSave, onDelete, size = 'md' } = {}
         <div class="modal-panel__header">
           <h2 class="modal-panel__title" id="shared-modal-title">${title}</h2>
           <button class="modal-panel__close" data-action="close-modal" aria-label="${t('modal.closeLabel')}">
-            <i data-lucide="x" style="width:18px;height:18px" aria-hidden="true"></i>
+            <i data-lucide="x" style="width:16px;height:16px" aria-hidden="true"></i>
           </button>
         </div>
         <div class="modal-panel__body">
@@ -465,6 +465,22 @@ export function btnSuccess(btn, originalLabel) {
   }, 700);
 }
 
+/**
+ * Versetzt einen Button in den Lade-Zustand (Spinner, nicht klickbar).
+ * Gibt eine Cleanup-Funktion zurück, die den Originalzustand wiederherstellt.
+ *
+ * @param {HTMLButtonElement} btn
+ * @returns {() => void} cleanup
+ */
+export function btnLoading(btn) {
+  btn.classList.add('btn--loading');
+  btn.disabled = true;
+  return () => {
+    btn.classList.remove('btn--loading');
+    btn.disabled = false;
+  };
+}
+
 /**
  * Zeigt Fehler-Feedback auf einem Button (Shake-Animation).
  * @param {HTMLButtonElement} btn